1. Field of the Invention
The invention relates to a group III nitride semiconductor free-standing substrate used for crystal growth of light-emitting devices such as a light-emitting diode (LED) and a laser diode (LD) or electronic devices such as a high electron mobility transfer (HEMT), a method of manufacturing the same, a group III nitride semiconductor device having a group III nitride semiconductor layer grown on the substrate, and a method of manufacturing the same.
2. Description of the Related Art
A group III nitride semiconductor attracts attention as a material for light-emitting devices of green, blue or ultraviolet light, etc., as well as a material for electronic devices mainly used for high output. Conventionally, most of the group III nitride semiconductors, of which application as a device has been reported, are grown on a heterogeneous substrate such as sapphire or 4H-SiC via a low temperature GaN or AlN buffer layer or a high temperature AlN buffer layer. However, when the group III nitride semiconductor layer is grown on the heterogeneous substrates, high density dislocation is introduced into the group III nitride semiconductor layer due to a difference in a lattice constant or a thermal expansion coefficient between the heterogeneous substrate and the group III nitride semiconductor layer. Since the dislocation in the group III nitride semiconductor layer acts as a non-light emitting recombination center or a diffusion path of impurity, the problem arises in that desired characteristics are not obtained or that the characteristics deteriorate quickly in a device made of the group III nitride semiconductor with high density dislocation.
When a free-standing single crystal substrate formed of the group III nitride semiconductor is used, it is possible to avoid the above-mentioned problem which is caused by lattice mismatch or thermal expansion coefficient mismatch, and thus, a blue-violet LD and a blue LD, etc., in which a group III nitride semiconductor layer is formed using a GaN free-standing substrate, are practically used. Currently, the most widely used method of manufacturing a group III nitride semiconductor free-standing substrate is a method in which a group III nitride semiconductor layer having a thickness from several hundreds μm to several cm is grown on a seed crystal substrate formed of a crystal different from the group III nitride semiconductor, such as sapphire, SiC, GaAs or Si, using a metal organic vapor phase epitaxial method (MOVPE method), a hydride vapor phase epitaxy method (HVPE method) or ammonothermal synthesis, etc. However, when the group III nitride semiconductor free-standing substrate manufactured by the currently common method is used, there is a problem that characteristics of the obtained device are largely different depending on a position within a substrate surface.
A method of manufacturing this type of group III nitride semiconductor free-standing substrate is known in which Ti is deposited on a surface of a GaN thin film on a sapphire substrate of which surface is a C-plane and is then heat-treated to form a void structure in GaN, several hundreds μm thick GaN of which surface is a C-plane is grown thereon by the HVPE method and the sapphire substrate side from the void structure is separated (Void-Assisted Separation: VAS method) (e.g., see the non-patent literary document of Yuichi OSHIMA et al. Japanese Journal of Applied Physics Vol. 42 (2003) pp. L1-L3). In addition, another manufacturing method is known in which several hundreds μm thick GaN is grown on GaAs substrate (cubic crystal) of which surface is a (111) plane with a SiO2 mask having openings formed thereon, and the GaAs substrate is subsequently removed (e.g., see the non-patent literary document of Kensaku Motoki et al. Journal of Crystal Growth Vol. 305 (2007) 377-383).
Since the above methods are the crystal growth of the group III nitride semiconductor on the heterogeneous substrate, a crystal in contact with the heterogeneous substrate at the initial growth stage has very high dislocation density. The typical dislocation density at the initial growth stage is on the order of 109-1010/cm2. In accordance with progress of crystal growth, the dislocation density gradually decreases and the dislocation density on the surface after several hundreds μm thick growth is on the order of 106/cm2 which is a level capable of manufacturing a LD. The crystal growth is completed after the dislocation density on the surface is sufficiently decreased as described above, the seed crystal is removed and the thickness is subsequently uniformed by polishing front and back surfaces of the substrate, then, the outer shape is trimmed if needed, thereby manufacturing the group III nitride semiconductor free-standing substrate.
In the group III nitride semiconductor free-standing substrate obtained as described above, the dislocation density gradually varies along a thickness direction of the substrate. Strain is generated inside the substrate due to the variation of the dislocation density in a depth direction, which causes that crystal orientations are different in a plane of the surface of the free-standing substrate as shown in FIG. 12 due to the influence thereof. Here, FIG. 12 is a schematic view of a group III nitride semiconductor free-standing substrate, showing a state that an angle formed by a predetermined crystal axis (e.g., a c-axis) and a surface continuously varies in a substrate surface. As shown in FIG. 12, a relation among an angle θ1 formed on one side of a free-standing substrate 850, an angle θ2 formed in the middle of the substrate and an angle θ3 formed on another side of the substrate is θ1<θ2<θ3. Since step density on the substrate surface is different when the angle formed by the surface of the substrate and the crystal axis is different, growth characteristics such as a thickness, a doping concentration and a mixed crystal composition, etc., in the group III nitride semiconductor layer grown on the surface are also different. This causes the above-mentioned large difference in the obtained device characteristics depending on a position in the substrate surface.
A group III nitride semiconductor free-standing substrate 950 in which the crystal orientations on the surface are uniformed by spherically polishing the substrate surface as shown in FIG. 13 may be realized in order to solve the above problem. However, since the group III nitride semiconductor is hard, it is difficult to perform the spherical polishing with good reproducibility and good accuracy.
A method of obtaining a free-standing substrate without performing the spherical polishing is proposed in which a sapphire substrate with a surface inclined at 0.07°-20° in an a-axis or m-axis direction from a C-plane is used, a nitride-based semiconductor single crystal epitaxial layer is grown on the substrate and the epitaxial layer is subsequently removed from a heterogeneous substrate, thereby obtaining a nitride-based semiconductor free-standing substrate having a desired off-angle (e.g., see JP-A 2007-197276).